Coating compositions which have been used heretofore have been primarily organic-solvent based compositions, typically containing high-solvent concentrations of 40-50% by weight or more. Use of such solvent-based coatings have caused significant air pollution problems resulting from the evaporation of organic solvents into the atmosphere during cure.
One alternative to such solvent-based coatings that has been proposed is the use of "high solids" formulations. Theoretically, these proposed formulations would contain at least about 60% by volume solids, preferably at least about 85% by volume or higher, with the remainder being organic and/or water solvent. The term "solids" is used herein and in the appended-to claims to mean the portion of the composition which is not subject to evaporation during cure. The "solids" may or may not themselves be solid before cure. Typically, the "solids" in the described compositions include resins.
Heretofore, commercially acceptable high solids liquid coating compositions have not been produced. One reason is that it has not been possible to make high solids formulations having sufficiently low viscosities to provide satisfactory coatings flow. For example, attempts to utilize traditional polyesters having molecular weights on the order of about 2000 and higher have been found to produce formulations having viscosities which are too high to be useful at high solids levels for many coating operations. On the other hand, use of low molecular weight polyesters, while reducing viscosities to desirable levels, does not produce useful high solids compositions because of the loss of significant amounts of the polyester compounds during cure due to their relatively high volatility.
U.S. Pat. Nos. 3,384,586 and 3,492,231 disclose resinous compositions containing a polymeric resin such as a polyolefin, polyamide, acrylic, polystyrene, polysulfide, polyether, polyester, melamine resin, alkyd resin and the like in combination with a non-Newtonian colloidal disperse system comprising (1) solid metal-containing colloidal particles predispersed in (2) a dispersing medium and (3) as an essential third component at least one organic compound which is soluble in said dispersing medium, the molecules of said organic compound being characterized by a hydrophobic portion and at least one polar substituent. The present invention is distinguishable from these references by virtue of the fact that the references disclose the use of the non-Newtonian colloidal disperse systems in combination with fully polymerized resins, while the present invention relates to the combination of such non-Newtonian colloidal disperse systems with cross-linkable materials. The problems inherent in the preparation of compositions containing cross-linkable materials which have not been fully cured are not recognized in either of these references, and the solutions to such problems are not provided.
U.S. Pat. Nos. 4,436,855 and 4,443,577 disclose urethane coating compositions containing non-Newtonian colloidal disperse systems comprising (1) solid metal-containing colloidal particles predispersed in (2) a disperse medium of at least one inert organic liquid and (3) as an essential third component at least one member selected from the class consisting of organic compounds which are substantially soluble in said disperse medium, the molecules of said organic compound being characterized by polar substituents and hydrophobic portions. The former patent relates to two-component urethane coating systems which further comprise an organic polyfunctional isocyanate and an acidic ester of a phosphoric acid. The latter patent relates to one-component moisture curable urethane coating compositions which further comprise an isocyanate-terminated prepolymer.
U.S. Pat. No. 4,425,466 discloses coating compositions containing a partially methylated partially methylolated melamine and a polyfunctional compound containing on average at least two functional groups per molecule selected from the group consisting of carboxy, hydroxy, primary and secondary amide and urethane. The reference indicates that the coating compositions are stable at ambient temperature and capable of fast cure response at relatively low curing temperatures.